This is an old post but I hoped to clarify what solved this issue for me.
Try simply closing the Project Media window. Having it open when working with enormous projects and tons of text edits, even while it's on its own tab without updating life still caused incredible lag when inputting new text. Closing it solved the problem for me.
I too have experienced this, and on a system that should have absolutely no problem whatsoever. In this case I'm talking about my computer at work, which was built in 2023, and has 64 GB of RAM, an A4500 GPU, and a Xeon gold 6320R CPU. It will sometimes take two or three full seconds before a typed character will appear in the text window (that machine is currently running VP19 but will likely be upgraded to VP22 soon).
If the coding on the software doesn't make use of current or relatively current computer builds, it's the coding of the title editing software. It's rather dated.
New title software is often a feature request. Right now, it's not on the table. I use NBFX Titler Pro or, there is Boris FX Title Studio. But both are resource intensive and need pre-rendering to playback.